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Community and Q&A

AC vs. whole house dehumidifier

cyarsev | Posted in General Questions on

Hello, I am looking for some advice on a central AC system vs. a whole house dehumidifier.   My house is located in Upstate NY (zone 6A) it’s a two story, 1700 sq ft, 2×6 stick built home, built in 1998.  Nothing too fancy in terms of energy efficient construction, but it’s not horrible either.  Overall, the house and area is humid.  We have lots of shade, the entire house is surrounded by 100 foot tall white pines and there are wetlands a few hundred feet behind the house.

When we moved in four years ago we had lots of window condensation during shoulder seasons.  To rectify this, we have installed downspout extensions on all the gutters, a bathroom fan in each bathroom (one that runs most of the year at a low 50 cfm), a portable dehumidifier in the basement that is set at 55% and  runs about 8 months a year (about one full bucket per 24 hrs in the summer), and a kitchen hood that vents outside.  All of these helped and window condensation is all but gone, but our house is still routinely above 60% humidity inside.  As I type this now, it is 73 degrees inside with 83% humidity (the last two days have been uncommonly hot and humid for this time of year), but those temps and humidity levels are very common July-Sept.

Therefore, I am thinking of replacing our old/broken central AC system or purchasing a whole house dehumidifier.  With the AC unit, I am a bit afraid that we would have to make the house an ice box to properly dehumidify it.  Many days it is not too hot inside (low to mid 70’s), but the humidity is 70-80% inside.  Does that make a whole house dehumidifier make more sense?

What about AC units with “Dry” or dehumidify mode?  Are those only available in two stage units?

Or should I at first try running a portable dehumidifier on the main level in addition to the basement?

Any comments or thoughts would be appreciated.


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  1. Expert Member
    Dana Dorsett | | #1

    How are you measuring the indoor humidity?

    83% RH @ 73F is a dew point of 67F, which is at least a bit lower than the outdoor dew points of the past 24 hours, but still pretty high if the AC has been running.

  2. Jon_R | | #2

    > try running a portable dehumidifier on the main level

    That's what I would do. Plus check that there aren't any avoidable moisture sources.

    Central dehumidifiers are quite expensive and doing without dehumidification is a bad idea (comfort-wise) for most of the US.

  3. GBA Editor
    Martin Holladay | | #3

    Regardless of what type of dehumidifier you buy, and where you install it, you will still need air conditioning. The first defense against high indoor humidity (assuming you've already attempted source control) is an air conditioner.

    If your air conditioner can't solve the problem, I agree with Jon: Purchase a portable dehumidifier and install it on the main level.

    All that said, I suspect that your house has a source of moisture that needs to be addressed.

    For more information, see these two articles:

    "Preventing Water Entry Into a Home"

    "All About Dehumidifiers"

  4. cyarsev | | #4

    Dana, we do not have AC, so it has not been running.

    Martin, why do you state "Regardless of what type of dehumidifier you buy, and where you install it, you will still need air conditioning." Also, we have tried controlling all point sources of indoor humidity, and we cannot find any wet areas in the basement, not sure where else to look.

    1. Expert Member
      Dana Dorsett | | #5

      Zone 6A NY cooling loads are dominated by latent cooling hours (humidity). There is usually (but not always) at least some sensible load (temperature) during periods of latent load- it's not insane to install a half-ton window-shaker and run it when the humidity is high and daytime temps are headed north of 75F.

      A dehumidifier converts a latent load into sensible load, taking the recovered heat of vaporization of the water collected, turning it into heat. This raises the temperature, which may be fine in a basement but not the best solution in your living space.

      Long term you may want to invest in a mini-split heat pump, and run it on "DRY" or "DEHUMIDIFY" mode when the indoor RH is north of 65%. In that operational mode there is still some sensible cooling, but the ratio of latent to sensible is much improved over a normal cooling mode.

    2. Jon_R | | #7

      Have you had a blower door test done to verify air sealing?

  5. cyarsev | | #6

    Dana, thanks for your input. Is a mini-split better at latent heat removal than a 2 stage AC unit?

    1. Expert Member
      Dana Dorsett | | #8

      It depends on how finely tweaked the system design is and which 2-stage AC unit, but in general yes, a mini-split that has a DRY or DEHUMIDIFY mode will be able to maximize the latent cooling with the least amount of sensible cooling.

      If running a window AC unit running it at the lowest blower speed will usually deliver it's best latent / sensible cooling ratio. It won't remove as much moisture PER HOUR as it will at high speed, but it will remove more moisture PER DEGREE of sensible cooling. But with a modulating mini-split with a dehumidification mode it will usually run the compressor at a higher speed than it would just for sensible , and adjust the blower speed up to where the coils stay just a bit higher then the freezing point for maximal moisture removal.

      With a ducted 2-stage central system the air handler generates room-to-room pressure differences, which drives outdoor air infiltration. On humid days the additional infiltration increases the actual latent load. That doesn't happen with ductless mini-splits or window air conditioners.

      1. cyarsev | | #9

        My wife just sent me info from the first contractor we had come this morning to look at replacing our broken AC unit, or to consider a whole house dehumidifier, or mini-split.

        From the sounds of it, the contractor did not like the idea of a mini split to handle 1,700 sq ft and our open floor plan (the large main room on the 1st floor has very high vaulted ceilings). He also stated, that we already have all the duct work and piping in place from the old unit, so the install on a new AC vs. a mini-split would be much cheaper.

        He quoted 3 Lennox AC systems, the EL16CX1, 14ACX, and 13ACX. He recommended a 2.5 ton system, but we also asked him to quote a 2 ton system.

        Using various online calculators I have seen recommendations between 1.5-2.5 ton units for my house. We have pretty standard wall insulation, but good attic insulation (I added R-30 on top of R-38 that was original). And our basement is also insulated. What size would you recommend?

        I am just concerned that with just a new AC unit, would I be able to dehumidify on days like this? The house is only at 72 degrees, but humidity is over 80%. Wouldn't you have to put the AC to 68 or 70 to get it to run enough to actually dehumidify?

      2. Jon_R | | #16

        > a ducted 2-stage central system the air handler generates room-to-room pressure differences

        But how much depends on how well the system is balanced and if the interior doors are open. Fan/ducts induced inside-outside pressure differences and infiltration can be basically zero. Or the ducted system can create a slight positive pressure (recommended for summer), which *reduces* infiltration in unwanted areas (eg cracks in walls).

  6. Expert Member
    Dana Dorsett | | #10

    Are the same ducts being used by a hot air furnace?

    What is your heating system size/fuel/age?

    99% and 1% outside design temperature is...?

    The size/tonnage of the now-defunct AC system?

    Most online freebie load calculators oversize by 25% or more, even when being fairly aggressive about the inputs. If one assumes they're 25% oversized (and not more), the 1.5 tons becomes 1.2 tons, the 2.5 tons becomes 2. I'm more inclined to believe the 1.2 ton number.

    A dumb rule of thumb for AC (often rong but usually not too far off), a ton per 1500'. That would put a 1700' house in the sub 1.25 ton range. Unless there's a lot of unshaded west facing window a 1.5 tonner is probably pretty safe.

    A dumb rule of thumb for 2x6 /R19 construction with an insulated basement at -5F or -10F would be 12-14 BTU/hr per square foot which would put a 1700' between 20-25,000 BTU/hr. If you have a heating history on the place it's easy put a stake in the ground on the approximate load using the existing equipment as the measuring instrument, using this methodology (WINTERTIME fuel use only):

    But paying an engineer or RESNET rater to run an aggressive Manual-J is still the right thing to do, for both the AC and heating numbers.

    If currently heating with oil or propane, heating with a heat pump will cost less, if sized correctly for the heating load (even a ducted heat pump with heat strips for backup at the coldest temperatures.) A modulating mini-split solution sized for the heating load won't be ridiculously oversized for the cooling load, and even at 2x oversizing would be able to dehumidify if it has a dehumidification mode.

    When it's 80% RH @ 72F indoors it probably would take setting the thermostat to 70F to pull the humidity down to 60%. But if the AC were being used more regularly the humidity wouldn't creep up to such a high level to begin with. In the short term a $100 half ton window air conditioner set to 65F running at low fan is probably going to be enough.

    A 1.5 ton mini-ducted Fujistu 18 RLFCD heat pump would likely cover your cooling load, and has a dehumidifcation mode. If the previous air conditioner's ducts were sized for a 3-4 ton system they're probably low enough in static pressure with the smaller air handler to not run into flow problems. But it's probably not enough heat pump to heat the place, and there are no provisions for auxilliary heat strips.

    A 2 zone Mitsubishi MXZ-2C20NAHZ or 3C24NAHZ or 2C30NAHZ with a 1.5 -2 ton MVZ air handler for the main zone using the existing ducts, plus an FH06 head for one zone could probably do it. The MVZ air handlers have auxiliary heat strip options, and running just the FH06 head in DRY mode when there is little or no real sensible load would probably dry out the place without over-cooling. It's not a 2-stage- it's multi-zone, but would be able to use the pre-existing ducts for both heating & cooling. The -NAHZ versions have specified output down to -13F outdoor temps, and decent capacity at 0F. The 3-C30NAHZ compressor is good for 28,600 BTU/hr @ +5F. The 2-ton MVZ-A24AA7 air handler is good for ~27K heating when there's enough compressor behind it, the 1.5 ton MVZ-A18AA7 is good for ~20K, which might be enough if the other zone wall-coil head is optimally placed.

    Of course that's a lot more money than just a 2-stage AC replacement, but if heating with propane or oil it would probably "pay off". Heating with a multi-stage cold-climate heat pump with HSPF of ~9.5-10 with 20 cent electricity in a climate zone 6 location would cost about the same as heating with oil at $2.50/gallon, instead of the $3+ current pricing. It's an even bigger difference when comparing it to propane.

    1. cyarsev | | #11

      Yes, the same ducts are being used by the hot air furnace.

      Furnance is propane, 5 years old, high efficiency unit. I will have to look up model and size later tonight. Propane is pretty cheap around here, 2 years ago it was $1.30/gallon, this past winter about $1.70, so much cheaper than oil.

  7. Expert Member
    Dana Dorsett | | #12

    $1.70/gallon propane is MUCH cheaper than the NY state average- are you sure that's right? (Are you sure it wasn't $2.70?)

    Propane: 91600 BTU/gallon at 95% combustion efficiency = 87,000 BTU/gallon delivered to the ducts (with some additional electricity use for the air handler & controls, etc). Normalizing that to gallons per MMBTU (million BTU), that's 1,000,000/87,000= 11.5 gallons/MMBTU

    At $1.70/gallon that costs $19.55/MMBTU.

    At $2.70/gallon (which is still below the state average) that costs $31.o5/MMBTU

    At this week's $2.96/gallon state average that costs $34.04/MMBTU

    Heat pump: Assuming an as-used HSPF of 9.5, that's 9500 BTU/kwh (and no additional power for the air handler, which isn't separated out in an HSPF test), so normalizing to MMBTU, that's 1,000,000/9500= 105 kwh/MMBTU

    At 20 cents/kwh that costs $21/MMBTU.

    So against $1.70 propane 20 cent electricity an HSPF 9.5 heat pump is pretty much a wash, about 7.5% more expensive, but really less than that if one factors in the power used by the propane burner.

    Against the NY average price for propane it's 38% cheaper, which adds up to a real chunk of change over the 15-25 year lifecycle of the equipment.

    Propane pricing is volatile, with very large year on year swings, and is subject to many market forces. Electricity is a regulated industry, and has over the longer term gone down in price in real terms, despite recent local/regional upticks. As more PV and wind goes on the NY grid at effectively zero marginal price the wholesale prices of electricity are again falling and over the long term will exert continued downward pressure on pricing.

    I can easily see propane prices doubling from $1.70 to $3.40 (it's exceeded that mark multiple times in the past 10 years), but if retail electricity doubles from 20 cents to 40 cents, even rooftop solar with no subsidy would be an absolute bargain that people would jump on, which will drive prices back down. Even if you keep the fossil burner, rather than a fancier 2-stage cooling-only unit. consider installing a heat pump (even a single-stage 1.5-tonner) as a hedge against propane price volatility.

  8. cyarsev | | #13

    OK, here are more details. I am part of a good buying club for propane, I paid $1.71/gallon in April of this year and $1.87 in January. I use about 800 gallons/yr between the furnace and hot water heater. The hot water heater is a 40 gallon tank unit, the energy star sticker on it says it uses 270 gallons per year. We use have the temp set low and use low flow shower heads, so lets say 230 gallons per year. That means the furnace uses 570 gallons per year (we keep our house cool).

    The furnace is a RUUD, 60K BTU unit, model RGRS-06EMAES. The energy star sticker on it reads a 92.5 AFUE.

    I have no info on the broken AC unit, other than the outside condenser says "Coleman Evap" The inside unit on top of the furnace is a Goodman, model U-30, part #15346-0, R22 refrigerant.

    Finally, I just measured the large room on our main floor. It is 13.5 x 33 ft (445 sq ft), but average ceiling height is 12'7". That is 58% more cube than standard 8 ft ceilings, so it effectively adds 258 sq ft to the house size, so lets say 1958 sq ft instead of 1700.

    1. Deleted | | #14


    2. christopherw | | #15

      I dug into this a few months ago, and the pricing seems to match the experience others have claimed when it's a self-owned, larger tank (like 500 gallons or more). It seems many folks are locked into situations where the propane company owns the tank (and sometimes refuses to remove it.), and they're therefore locked into steadily, slowly increasing prices.

      1. Expert Member
        Dana Dorsett | | #18

        They'll ALWAYS remove it for you, for a price!

        The whole micro-monopoly aspect of the residential retail propane biz is rife with abuse. I know a retired woman on a fixed income in a tiny house in MA who was being nicked over $5/gallon and kept paying- though she could usually scrape up enough to fill the tank, she didn't have the cash funds for the removal fee, which was well north of a grand. That went on for several years before she sold the house and moved to Arizona.

        I read of a situation where a guy in NH installed a ground source heat pump system and converted to all-electric on the appliances. The propane vendor at first accused him of picking the lock and letting somebody else fill the tank. When the situation was explained the vendor then demanded the $2K removal fee specified in the prior agreement, and was pretty insistent about either start buying propane or pay to have the tank removed immediately. Since there was no language in the agreement for a minimum annual consumption, and nothing stating that it had to be removed if not in active use, the tank remained, abandoned in place for years. For all I know it's still there.

    3. Expert Member
      Dana Dorsett | | #19

      If you're only burning 570 gallons/year for space heating a 60K furnace is oversized, but without EXACT fill-up dates & quantities, your ZIP code (for weather data) and your thermostat settings it's hard to say with any accuracy what your actual heat load is, or what the minimum NY-legal size would be.

      The 570 gallons burnt at 95% delivers only 50MMBTU. In a 8000 heating degree day (base 65F) in the middle of zone 6 would be 50M/8K= 6250 BTU per degree-day or (/24= ) 260 BTU per heating degree.

      With an outside design temp of -10F and an indoor design temp of 60F that would be 70F heating degrees and a heat load of 260 x 70F= 18,200 BTU/hr. At an indoor temp of 70F it would be 80F heating degrees, and a heat load of 260 x 80F = 20,800 BTU/hr.

      Of course there is a lot of distortion in the estimate due to using a 65F base temp, and we don't have the real thermostat settings, weather data & outside design temp for the location, but it's unlikely that the 99% heat load at an indoor temp of 68F would be more than 30K.

  9. joshdurston | | #17

    One possible negative to ducted AC is being forced to oversize the AC to min CFM of the furnace. Sounds like you only need about the smallest conventional split AC unit (usually 1.5ton), and your actual cooling load might be smaller. But your check your furnace's minimum CFM, it may necessitate going with a bigger AC.

    I have exisiting ductwork but am in the process of installing a minisplit in my bungalow. Furnace CFM is too high, and I want the cooling upstairs where I need it, not leaking out of my crappy ductwork into my already cool basement. I run a portable dehum in the basement to keep it under 60%rh. An ancient portable 9000btu AC unit keeps my house comfortable, but the new mits FH09 will be a massive step up in efficiency and comfort and noise levels.

    1. Jon_R | | #27

      As Josh suggests, proper CFM/ton is far more important to latent removal than the shorter cycles that occur with oversized AC. The former effect is huge and the latter is quite minimal.

  10. cyarsev | | #20

    Zip code is 12833, 99% temp is -4 for Glens Falls (the closest city listed). Thermostat setting is 60 at night, 63 during the day.

    1/27/17 filled tank with 295.3 gallons
    5/17/17 filled tank with 183.9 gallons
    They normally always fill the tank to 80% full. Therefore, I assume, but I think correctly, that we used 183.9 gallons between 1/27 and 5/17/17.

    1. Expert Member
      Dana Dorsett | | #23

      Nothing from this past heating season?

      For a 2x6 house assume the heating/cooling balance point is about 8-9F below the room temp. So we'll use base 53F data, and work from there, then add 5F to the heating degrees number to reflect the load at +68F indoors instead of +63F.

      The fact that much of that period is after the spring equinox means there's a significant solar gain error, but that's offset by the hot water use error.

      The nearest weather station that I could find with continuous data for that period was KNYCORIN5 in Corinth. Summing the 1/27 - 5/16 (the day before the fill-up) then 1/28 - 5/17 (starting the day after the initial fill-up) would normally average out the time-of day HDD error but since the last fill up was in full spring and the first was in winter we'll just use the 1/27 - 5/16 numbers, which added up to 1803 HDD53

      184 gallons at a net 87,000 BTU/gallon becomes 16,008,000 BTUs, which divided by 1803 HDD is 8879 BTU/HDD, or 370 BTU per degree hour.

      At an outside design temp of -4F and a balance point of 53F that's 57 heating degrees. Since we're calculating the load at a temperature 5F warmer than you kept it, that becomes 62F heating degrees.

      The implied load at at -4F indoors, +68F indoors is then 370 BTU/F-hr x 62F = ...

      >>>>>> 22,940 BTU/hr <<<<<<

      Call it 23K

      That's only 15% higher than my WAG estimate in the early post (at a colder design temp using the WAG on furnace-fuel only), and a very credible number. For a 1700' house that's a ratio of 13.5 BTU/hr per square foot of conditioned space, which is consistent with expectations. A ratio of 11-13 BTU/hr per square foot at 0F is typical for a tight 2x6 framed house, it'll be ~7% higher than that at -4F so a ratio of 13.5 BTU/hr @ -4F is definitely in the expected ball park.

      If the solar gain error is bigger than the hot water use error the real heat load will be higher than 23K if the hot water use error is bigger than the solar gain error it the real load is lower. That's why it's better to use wintertime only data, which is when the hot water use is a smaller fraction of the total load, and the solar gains are lower too. But we can say the real heat load is most likely between 20-25K, not more.

      The output of the 18RLFCD @ -5F is about 19-20,000 BTU/hr, so it's not going to cover the entire heat load, but in combination with a 9RLS3H wall-coil type in the bigger open space you'd be more than covered, since the 9RLS3H good for another 11,000 BTU/hr even at -15F. (At +5F it's good for 15K.) You'd have ample capacity relative to the whole-house load at -5F, and even though the 18RLFCD doesn't have a specified capacity below -5F you'd surely be covered at -10F (which only adds another ~2200 BTU/hr to the whole house load over what was calculated.) The 18RLFD doesn't stop cold at -6F- it keeps on chugging no matter how cold it gets, but the capacity isn't specified below -5F.

      If the duct design works out and can be balanced to put less in to the zone covered by the 9RLS3 to be able to keep the other rooms warm enough this looks like a realistic prospect. Without any signficant duct design changes the 18RLCD installation should come in under $5K, a 9RLS3H is usually under $4K sometimes only $3K . That's going to be cheaper and more comfortable than a Mitsubishi MVZ + FHxx wall coil multi-split solution

      The minimum output at 47F of either is 3100 BTU/hr at 70F indoors which is only 16F below your usual 63F room temp, and only 6F below the presumptive 53F balance point. At 370 BTU/F-hr it takes 3100/370= 8F heating degrees for it to start cycling on/off, so odds are with just one running it will start cycling a bit whenever it's 45F or warmer, and with two running that'll start when it's in the mid to high-30s outside, so it'll be more efficient to run only one or the other when it's 40F or warmer, but fine to run both when it's below freezing out.

  11. cyarsev | | #21

    I just looked up the specs on my furnace for CFM info. This is what I found.

    Heating C.F.M. @ .2" [.049 kPa] = 845 [398]

    Cooling C.F.M. @ .5" [.124 kPa] = 1100 [519]

  12. cyarsev | | #22

    Dana, earlier you mentioned a 1.5 ton mini-ducted Fujistu 18 RLFCD heat pump. I have been reading about these and looking through their website. These seem like they might make sense for my application. I have a very large main room, that is very open, and connects directly to the upstairs bedrooms via vaulted ceilings in the house that a single head unit would work well in.

    It is my understanding from you and their website, that these are variable speed units with a dry mode, and therefore would do a better job at dehumidifying under high latent heat and low sensible heat conditions (mid to high 70's temp, with 70-85% RH), of which we have many days of per year, compared to a traditional single stage 1.5 or 2 ton AC unit.

    I also see that they make bigger versions like the 24 RLB that might even help me out more with heating to help hedge against propane prices.

    Finally, I see prices online for the unit just under $2K, so an installed price might be around $4k, that seems reasonable, as traditional AC unit quotes are coming in around $3 - 3.8K.

    Thoughts on all this?

  13. cyarsev | | #24

    Dana, I just ran the calcs again using 2018 data and got a substantially different number of 28,976 BTU/hr. Some of this is hot water use, but that was in the prior calculation you did too. Any thoughts?

    1/16/18 Filled tank
    4/20/18 Filled tank with 262.9 gallons
    HDD53 = 2039.2 for Corinth station

    Also, a few more questions. When I state my house is 1700 sq ft, I did not include the basement. Should I be including it? The basement is finished and heated, but I close most of the ducts down there as we don't use the space that much.

    In order to tie in a mini-split into my pre-existing duct work, I assume it has to be "slim duct" style like the 18 RLFCD. Do any other companies make a similar style in 24,000 btu?

    Do these furnace specs help at all with knowing if the mini-split will move enough air for the duct work?

    Heating C.F.M. @ .2" [.049 kPa] = 845 [398]

    Cooling C.F.M. @ .5" [.124 kPa] = 1100 [519]

  14. Expert Member
    Dana Dorsett | | #25

    Unless you're actively heating the basement and it's mostly above-grade (a heated conditioned walk-out), it doesn't really count when using heating/cooling sanity-checking rules of thumb.

    I'm not aware of any manufacturer with a 2-ton mini-duct mini-split. Most vendors don't even use vapor-injection type compressors that give it better low-temp performance/capacity on their dedicated mini-duct mini-splits. Mitsubishi's "hyper heating" multi-splits with an MVZ air handler is your next step up. They don't modulate, but if right sized they're not bad.

    1. aunsafe2015 | | #26

      Does the Mitsubishi PEAD count as a mini duct mini split? I think they have 2+ ton models.

      Also, do you know if the Fujitsu mini splits in dry mode let you select a set point? My Mitsubishi FH09 does not let me select a set point in dry mode and eventually makes me too cold.

    2. cyarsev | | #28

      Hi Dana, I have had a few contractors come in and look at our house. They all can't get past the fact that we already have duct work, they keep saying "Why would you do a mini-split when you have duct work?"

      I explain that I like how Mini-splits have a "Dry" mode and they are more energy efficient. However, they all are recommending a 2.5 ton single stage AC unit instead.

      A few questions.
      1) Will dry mode on a mini-split do anything when outside temps are in the 60's, but still humid, or will it cool the house too much?
      2) With my 1700 sq ft home (but more like 1950 sq ft when taking into account the volume of the vaulted ceilings in our large main room), standard r-19 walls, do you still think a 1.5 ton AC unit will do? These contractors might go along with a 2 ton unit, but will probably laugh at a 1.5 ton.
      3) With our rough history based calculations of heating load being between 23-29K BTU's/hr (including hot water heater use), does that tell us anything about cooling load? I would assume cooling load would be much less, as the Delta T is way lower when cooling.


  15. Expert Member
    Dana Dorsett | | #29

    1> When there's no sensible cooling load to deal with DRY or DEHUMIDIFY mode on a Mitsubishi or Fujitsu will overcool the house.

    2> "In Manual-J we trust".

    I don't care what a mere HVAC contractor or somebody on the internet says, and you shouldn't either. Run the numbers. A ton per 1000-1500' is pretty typical for house that size, but nobody lives in the "typical" house, they live in an "actual in-situ real world" house with lots of factors affecting the load.

    Your cooling load might be 2.5 tons, it might be 3/4 ton, but until someone takes the time to measure it up and run the REAL numbers it's all a WAG. My 2400' 2x4 framed 1.5 story antique has peak cooling load under 1-ton, due to deep overhangs on windows and favorable afternoon shading factors. YMMV.

    3> Heating loads have almost no bearing on cooling loads. A large fraction of the cooling load is direct solar gains through windows, and in your area the latent load fraction is also pretty real. The indoor to outdoor temperature deltas still matter, but it's not the overwhelming factor the way it is with heat loads.

  16. cyarsev | | #30

    Can anyone shed some light on the differences and recommend either the Fujistu 24RLXFW1 vs. the 24RLB? The RLX is energy star rated, SEER is 19.5 vs. 18. Both have HSPF of 10.6. The RLX is rated for heating down to -5 vs. +5 for the RLB.

    The RLX rated power is 1.76 KW cooling and 2.38 KW heating.
    The RLB rated power is 2.4 KW cooling and 1.93 KW heating.

    I will be using this for cooling and as a heating supplement, I still have a 60K propane furnance for really cold weather. Should I save some money and go with the RLB or is the RLX worth it?

  17. Expert Member
    Dana Dorsett | | #31

    Without making me re-read this entire thread, did we figure out what your space heating load at +17F, +47F and at your -4F outside design temp?

    The 24RLB has a much wider modulation range in cooling mode, dropping all the way down to 3100BTU/hr at min-mod, compared to 9,900 BTU/hr for the 24RLX. Your whole house cooling load might not be much higher than the 9,900 BTU/hr- you'll get a lot longer cycles (and probably more drying) out of the 24RLB.

    In heating mode the minimum @ +47 is the same 7500 BTU/hr for both. But the max heating capacity of the 24RLX is substantially higher at 36K instead of 27K.

    A pretty-tight 1700' house with 2x6/R19 type construction should come in between 20-25,000 BTU/hr @ 0F for heating if it has an insulated foundation, a bit more if the basement walls are not insulated. (How much more depends on how much above-grade exposure there is on the foundation.)

    Digging up the extended temperature capacity tables for the two models would be useful. According to the table on p.25 of the brochure the 24RLXFW1 is good for 94% of it's nominal 25,200 capacity at -5F (that would be 23,700 BTU/hr). The similar table on p29 doesn't specify the output of the 24RLB @ +5F.

    So the 24RLX can pretty much cover your whole-house load if your design temp is in negative single digits, but without capacity tables or an more accurate heat load estimate for your house it's hard to say how much the 24RLB would cover. We only know it's "nominal" heating output which is the modulation level it was tested for efficiency at +17F & +47F per AHRI protocols, but we have no idea what it's max capacity is at it's specified +5F minimum operating temperature.

    So it comes down to whether you're buying it primarily for heating vs. primarily for cooling.

    1. cyarsev | | #33

      The two stake in the ground heat measurements from 2017 and 2018 came it at 23K and 29K including hot water use for my house. My area design temp is -4.

      For the 10.8 cents/kWH I pay for National Grid electricity it seems to make sense to use this for cooling AND heating.

      1. Expert Member
        Dana Dorsett | | #36

        According to line 628, column AM of the 10/30/2018 updated NEEP spreadsheet at +5F the AOU24RLXFW1 is good for 25,220 BTU/hr.

        That's more than the 24RLB's nominal heating output at +17F, and probably close to (possibly slightly more than) your heat load at +5F.

        The max output of the 24RLXF1 at -5F is 22,55o BTU/hr, which is less than your whole-house heat load at that temp, but probably sufficient to cover that zone, or almost cover it.

        Even though it's cooling modulation range doesn't go low enough to avoid cycling, the RLX is the right choice here. It will have to run in dehumidify mode more often than the RLB, but it has the capacity and higher low-temp efficiency sufficient to make it the more appropriate choice. Even if you run out of propane or the propane burner gives up on you during a cold snap there will be at least part of the house fully up to temperature. With the RLB that would not be the case.

        In a tall room it will be worth using a wired or wireless wall remote set up to use the temperature sensor in the remote rather than in the ductless head for monitoring room temperatures. The defaults are set up to sense room temp via the incoming air at the head, so this is a set-up issue, more important in your case than most mini-split installations. I relatives heating/cooling their whole house with an FE18 Mitsubishi mounted halfway up the wall in the Great Room. Before installing a remote thermostat they were tweaking the temps nearly every day to keep it tracking as the weather changed.

  18. walta100 | | #32

    “I will be using this for cooling and as a heating supplement, I still have a 60K propane furnance for really cold weather. Should I save some money and go with the RLB or is the RLX worth it?”

    Chris from a dollars and cents point of view in most locations given the local prices of propane and electricity the heat pump will cost much less to operate. The problem is from a comfort point of view most people will prefer the propane furnaces comfort. The propane furnace will slowly blow 130° air from the registers. The heat pump will move twice as much air at 85°it will feel cold and drafty by comparison. If both systems are available, only occupants with wills of iron are going to operate the heat pump. You could start some big arguments should your spouse disagree.


    1. cyarsev | | #34


      Thanks for the good insight. I have never heard of this problem before. We were planning on installing in the large main room, that is the main living area. Might be too drafty to be comfortable?

  19. Expert Member
    Dana Dorsett | | #35

    Unlike traditional 1-2 stage ducted split systems, the output temperatures of mini-splits are only tepid when there's almost no load and operating at minimum modulation, or when they are maxed out, operating near or below their minimum operating temperature. See the exit air temperatures at various operating conditions in Tables 5 & 6 of Appendix A in this document:

    At minimum blower speed minimum modulation the output is tepid, but the air velocity is low.

    At outdoor temperatures & load conditions that matter, think "warm summer breeze", above body temperature, usually in the same range as hydro-air, or condensing propane or natural gas.

    At your heat load & outdoor design temp go with the 24, even though it means you'll have to run it in DEHUMIDIFY mode more often than you would with the 24RLB during the cooling season to keep indoor humidity bounded.

  20. walta100 | | #37

    Please do not get me wrong properly installed heat pump should not be uncomfortable but does take some time to get use to a heat pump. In a side by side comparison like what I think you are considering it to me seems unlikely the heat pump will get chosen very often.

    If you do have both be sure to post back with your feeling and how often you used what system.


  21. cyarsev | | #38

    I just ran the numbers at after measuring all the walls, windows, doors, etc of my house. At a -4 design temp and 65 indoor heat temp and a 85 cooling design temp and a 72 cooling temp here are the numbers it calculated.

    Heating BTUs = 32,622 (this is quite a bit higher than the 23K and 28K we calculated using the propane usage and degree day history, and those estimates included hot water heating too, so they should have been a bit high). One thing that might account for this, is on the on-line calculator I could not figure out a way to account for my attached garage. It is not heated or insulated, but surely it has to have some insulating effect, as it covers nearly the entire East wall of the house. In the winter it might get down to 25-30 inside the garage, when it is zero or negative outside.

    Cooling BTUs = 13,436. Sensible =11,663, Latent = 1,773. I was surprised at how low the latent heat was. It seems to me we have many days not much warmer than 72 inside my house by the humidity is crazy high.

    Do these numbers make sense? It sure seems like a 2.5 ton AC unit that all the contractors are quoting is way too big. Makes me want to buy a one ton window unit in the great room and try it. It's a cheap experiment, compared to 3-4K for a new unit.

    Also, the 60K furnance does not seem way oversized, especially if I actually wanted to heat my finished basement. We don't use the space much, so we close off most of the vents down there to keep it cool.

    1. Expert Member
      BILL WICHERS | | #39

      2.5 tons of AC is 30,000 BTU. Probably way oversized regardless of the sensible/latent mix if your other calculated numbers are at all close. Oversized AC units will do a crummy job of dehumidifying resulting in a cold, muggy house.

      Your 28k number is pretty close to your ~33k number. You might be close there. A 60k furnace seams oversized to me, but you can get multistage and modulating units that would give you some future proofing for your possible basement heat needs, while limiting the short cycling issue you’d have with a too-big unit.


      1. Jon_R | | #42

        > Oversized AC units will do a crummy job of dehumidifying

        See below, figure 5-45, showing that the effect is quite small, even when capacity is 10x the current load. And at 72F outside, any AC will do a crummy job. Because it won't run - you need a dehumidifier.

    2. Expert Member
      Dana Dorsett | | #40

      Online calculators such as coolcalc & loadcalc always have a double-digit error to the high side. Reducing the presumptive air leakage and ventilation losses to zero (which can be achieved with air sealing & HRV) gets it a lot closer.

      Th sensible cooling load of about 1 ton sounds correct. The calculated latent load is largely a function of ventilation/infiltration rates, but may not accurately reflect indoor humidity sources. Yes, a 2.5 ton air conditioner is grossly oversized at about 2x, but a mini-split that can modulate down to 7500 BTU/hr won't be.

      The ~57K output of a 60K- furnace is still 1.75x oversized for the coolcalc calculated load (which is likely higher than reality, as determined by fuel use), and will likely still more than 1.4x oversized even if actively heating the basement. Basement loads just aren't very big unless the foundation walls are completely uninsulated and there is 2' or more of above grade average exposure on the perimeter. Since you have the coolcalc mostly teed-up, enter the data for the basement, see what it comes up with. How cold does it actually get down there when it's -4F outside?

      Applying ASHRAE's 1.4x multiplier to the 32,622 BTU/hr it comes to 45,671 BTU/hr, which is less than the output of a 95% efficiency 50K condensing furnace. Applying the 1.4x factor to the high-side 28K number calculated from fuel use comes to 39,200 BTU/hr.

      Applying the AFUE testing presumption of 1.7x to the 32,622 BTU/hr comes to 55,457 BTU/hr, slightly less than the output of a 60K condensing furnace. Applying 1.7x to the 28K number comes to 47,600 BTU/hr roughly the output of a 50K furnace.

      So while a 60K furnace isn't grossly oversized, a 2-stage 50K condensing furnace would be somewhat more comfortable, and still have the load covered well into negative double-digits.

      1. cyarsev | | #41

        I don't think the basement is ever under 50 degrees, probably more like 55, even when it is -20 outside. Only about 12-18" of the basement walls are exposed above grade and the walls are insulated with 2" fiberglass.

  22. Expert Member
    Dana Dorsett | | #43

    If it's 55F indoors when it's -20F outside that's the same delta-T as when it's 71F indoors and -4F outside, which is an indication that the heat load of the basement is pretty small, probably less than a 10% adder to your 32.6K number, or less than the difference in fuel-use heat load calculation and the coolcalc Manual-J. (32.6K/28K= 16.4% difference.) It's still looking like a 50K furnace.

    The 2" fiberglass is about half current code minimum, but way better than nothing. For yuks let's call the concrete + 2" fiberglass R8, and assume 2' of above grade exposure instead of 1-1.5' to account for the slab and ground losses, and a perimeter length of 125'. That's 250 square feet. At a delta-T of 71F (-4F outdoors, 68F indoors) that's a load of 70F x 250'/R8= 2189 BTU/hr. There will be air infiltration (most of which is already accounted for in the fuel use numbers) and some window losses, but I doubt the true basement losses are more than 3000 BTU/hr let alone 5000 BTU/hr, which would still not be enough to rationalize upsizing to a 60K furnace.

    The LOW fire output of a typical 60K condensing furnace is ~40,000 BTU/hr, which is above your highest calculated heat load. Ideally whatever gets installed would have a minimum fire output LESS than your calculated heat load, with a high-fire output that's above your calculated heat load. Most 2 stage 40K condensing furnaces would fill that bill, even if you randomly tacked on 2-3K of load to be sure the basement was fully covered. A 95% efficiency 40K will deliver 38,000 BTU/hr, 5000 BTU/hr more than the coolcalc numbers.

    If your heat load is the coolcalc's 33K at a delta-T of 70F that means the heat load is growing by about 470F per degree. With 5000 BTU /hr of additional capacity beyond the calculated load that means you're good for another 5000/470= 11F below the design temperature, or about -15F before it starts to lose ground.

    If cooling with an appropriately sized (for cooling) 1-ton or 1.25 ton cold climate mini-split such as the Fujitsu 12RLS3H or 15RLS3H you'll have another 11,500-16,000 BTU/hr of "Hail Mary" auxiliary capacity at -15F to work with for the Polar Vortex events. That brings your total heating capacity up to the 50K range or higher, making anything more than 40K of condensing furnace overkill.

    Basement loads don't track those of upper floors very closely, and during the shoulder seasons may still have a heating load when the upper floors have a modest cooling load. Just extending the furnace ducting for the basement zone is less than ideal. Basement zone cooling loads in your area would be latent-only. If the water heater is a propane fired tank, a right-sized hydronic coil for the basement zone running off the water heater is a better solution than tapping off the central furnace duct work.

  23. cyarsev | | #44

    Hello, I just wanted to update this post with more info. I have winter time only numbers for heating now from last winter, they are below. The 19K BTU/hr seems very low, not sure why they are so much lower than the other numbers we calculated.
    12/19/18 thru 3/7/19 (54 days)
    279.5 gallons used (including hot water usage)
    Degree days
    12/19-3/6 = 3307
    12/20-3/7 = 3318
    Avg = 3312.5
    279.5 gallons*87000 btu/ gallon = 24,316,500/3312.5 hdd = 7340.83 btu/hdd = 305.87 btu/ hd hour x 62 f = 18964 btu/hr

    Also, this summer I purchased a 12,000 btu window air conditioner for our great room on the main floor as a $400 dollar experiment, before I went and replaced the broken central AC for $3 or $4K. It easily cools the entire first floor to 72 degrees with a very light duty cycle on the low fan speed. If I lowered the temp a bit and kept it running longer I believe it could pretty much do the whole house. This pretty well agrees with my numbers of 13.4K for cooling. I guess if I ever replace the central AC, I certainly do not need a 2.5 or 3 ton unit.

  24. cyarsev | | #45

    I wanted to correct my above reply, I figured out why I was calculating 19K btu/hr vs. my previous estimates that were higher. When I pulled the HDD of 3312.5 I pulled them at 65 degrees, instead of 55 (avg house temp was 63). Correcting for this, the new HDD was 2469.5.

    Updated Calcs below:
    12/19/18 thru 3/7/19 (54 days)
    279.5 gallons used (including hot water usage)
    Degree days at 55 = 2469.5
    279.5 gallons*87000 btu/ gallon = 24,316,500/2469.5 hdd = 9,846.73 btu/hdd = 410.28 btu/ hd hour x 62 f = 25,437.39 btu/hr

    Also, this summer I purchased a 12,000 btu window air conditioner for our great room on the main floor as a $400 dollar experiment, before I went and replaced the broken central AC for $3 or $4K. It easily cools the entire first floor to 72 degrees with a very light duty cycle on the low fan speed. If I lowered the temp a bit and kept it running longer I believe it could pretty much do the whole house. This pretty well agrees with my numbers of 13.4K for cooling. I guess if I ever replace the central AC, I certainly do not need a 2.5 or 3 ton unit.

  25. cyarsev | | #46

    Here is an update from this heating season. In the fall I replaced my propane hot water heater with a electric heat pump unit. My furnace is now the only propane usage in the house, so this year's numbers should pretty accurately reflect actual heating load.

    Propane usage between 11/9/2020 and 2/10/2021= 260.3 gallons
    Thermostat = 60 at night, 65 during day
    Assumed balance point is 8 degrees below room temp, therefore assume Base temp data =55
    HDD55 between 11/9/20-2/10/21 =2470.5 (used Glens Falls station data)

    260.3 gallons x 87,000btu/gallon = 22,646,100

    22,646,100/2470.5 = 9,166.61 btu/HDD
    9166.61/24 hours = 381.94 btu/ heating degree hour

    Outside design temp of -4F and a balance point of 55F that's 59 heating degrees.
    Calculating the load at a temperature 5F warmer than the house (68 vs 63), that becomes 64F heating degrees.

    381.94 x 64 = 24,444.16 btus/hour for heat only

  26. cyarsev | | #47

    Here is an update from the winter of 2021-2022. My furnace is now the only propane usage in the house.

    Propane usage between 10/15/2021 and 5/15/2022= 415.2 gallons
    Thermostat = 60 at night, 65 during day
    Assumed balance point is 8 degrees below room temp, therefore assume Base temp data =55
    HDD55 between 10/15/2021 and 5/15/2022 =4483.6 (used Glens Falls station data)

    415.2 gallons x 87,000btu/gallon = 36,122,400 BTUs

    36,122,400/4483.6 = 8,056.56 btu/HDD
    8056.56/24 hours = 335.69 btu/ heating degree hour

    Outside design temp of -4F and a balance point of 55F that's 59 heating degrees.
    Calculating the load at a temperature 5F warmer than the house (68 vs 63), that becomes 64F heating degrees.

    335.69 x 64 = 21,484 btus/hour for heat only

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